The invention comprises a method for treatment of potato fruit water (potato waste water). Potato fruit water is a certain fraction which appears during the potato starch production. The disintegrated, clean potato chips are first introduced into a decanter, the supernatant therefrom being the potato fruit water; the residue is introduced into a centrifugal sieve, from which two fractions are taken out, i.e. the potato starch fraction and the potato pulp fraction. The annual production of potato starch on a global basis is at least around 10.sup.6 tons, and the corresponding volume of potato fruit water is at least around 4.times.10.sup.6 tons, with a dry matter content of at least 10.sup.5 tons.
Before the strict environmental regulations enforced today were passed, potato fruit water was simply fed into the sewer system or sprayed onto the fields. Another prior art attempt to solve the problem comprising the utilization of potato fruit water in an environmentally acceptable and economically sound manner has been established by utilization of heat coagulators with subsequent recovery of the insoluble part of the protein fraction by centrifugation as a concentrated protein precipitate. Also attempts have been made to use ultrafiltration of the potato fruit water in some potato processing plants. As around 50% of the total amount of the potato protein is low molecular and non-coagulable, the mentioned methods leave the non precipitable protein and the low molecular protein components in solution. Thus these unwanted products will be present in the waste water which has to be treated in waste water treatment plants or utilized by other means. After introduction of the enviromental regulations in many countries it became necessary to purify the potato fruit water as efficient as possible before it was introduced into the sewer system. This was a costly procedure, and the potato starch manufacturer could derive no benefit from this expense unless valuable products could be generated. It was suggested to concentrate the potato fruit water in order to use it as an animal fodder; however, as the potato fruit water without protein separation could not be concentrated more than corresponding to about 25% dry matter, due to a sharp viscosity rise, and as this percentage was considered too low for practical utilization of the concentrated potato fruit water as an animal fodder, this suggestion did never lead to a commercially sound utilization of potato fruit water.